Method of injection molding with displacement of mold from injection position and applying pressure during cooling

Abstract

A method is disclosed for molding plastic articles, wherein the method includes the steps of filling a mold cavity at a first station with heated flowable plastic material and then displacing the mold to a second station where the plastic material is cooled under pressure. The apparatus includes a rotatable support for displacing the mold between the first and second stations, a plastic supply means at the first station to fill the molds, and a displaceable rod at the second station for insertion into the mold to apply pressure to the plastic material during solidification cooling.

Having fully and completely described our invention, we now claim: 
     
       1. A method of forming a plastic article, comprising the steps of: filling at a relatively low injection pressure, less than about 5000 psi, an internal mold cavity of a mold at a first station with heated, flowable polyethylene terephthalate plastic material, the mold having a sprue opening;   moving the mold from the first station along a partial circumferential path to a second station in which the sprue opening is in alignment with a pressurizing rod;   at said second station, displacing the rod into an opening in the mold to apply pressure to the plastic material within the internal mold cavity; and   maintaining pressure on the plastic material within the internal mold cavity by said rod while the plastic material is cooled to a self-sustaining state to form an article with a reduction in flashing, with a reduction in heat loss at the first station and with a minimum of degradation of polyethylene terephthalate.   
     
     
       2. A method of forming a plastic article, comprising the steps of: filling an internal mold cavity and spure opening in a mold at a first station with plasticized plastic material;   moving the mold from the first station to a second station where the sprue opening is in alignment with an axially reciprocal pressurizing rod, the moving reducing flashing and reducing heat loss at the first station to decrease cycle time;   axially displacing the rod into the sprue opening to apply pressure to the plastic material while the material is still in a plasticized state within the internal mold cavity; and   maintaining pressure on the plastic material within the internal mold cavity by said rod, while the plastic material is cooled to a self-sustaining state.   
     
     
       3. A method of forming an article of thermoplastic material, by the steps of: flowing heated, flowable polyethylene terephthalate thermoplastic material into an internal mold cavity of a mold positioned at a filling station, thereby essentially filling the internal mold cavity with thermoplastic material at a first relatively low injection pressure;   while the thermoplastic material is still in a heated, flowable condition within the internal mold cavity, displacing the mold from the filling station to reduce flashing and heat loss at the first station;   inserting a pressurized piston into an opening in the mold and applying pressure that is greater than said first pressure to the thermoplastic material within the mold cavity while the thermoplastic material is cooled from the heated, flowable condition to an essentially self-supporting condition;   removing the thermoplastic material from the mold cavity in said self-supporting condition as a molded article; and   displacing the mold back to the filling station to receive additional flowable thermoplastic material.   
     
     
       4. In a method of forming an article of thermoplastic material, the steps of: accumulating a shot of heated, flowable thermoplastic material in an accumulator chamber;   flowing the heated, flowable thermoplastic material from the accumulator chamber at a low injection pressure, through a sprue opening in a mold and into an internal mold cavity in the mold, which is comprised of separable mold sections positioned at a filling station, thereby essentially filling the internal mold cavity and the sprue opening with thermoplastic material;   while the thermoplastic material is still in a heated condition within the internal mold cavity and the sprue opening, displacing the mold sections from the filling station to a second station where the sprue opening is in alignment with a pressurizing piston having a size and cross-section essentially the same as the sprue opening, the displacing reducing flashing and reducing heat transfer between the filled mold and the accumulator chamber at the first station;   inserting the pressurizing piston into the sprue opening and applying pressure to the thermoplastic material within the mold cavity while the thermoplastic material cools from the heated condition to a self-supporting condition;   removing the thermoplastic material as a molded article from the mold cavity at a decreased cycle time between the flowing and removing steps that minimizes the degradation of the material; and   displacing the mold sections back to the filling station to receive a subsequent shot of flowable thermoplastic material from the accumulator.   
     
     
       5. A method for forming an essentially tubular thermoplastic article, comprising the steps of: positioning a sectional mold at a first station, the mold including (a) a female portion defining a generally vertical elongated cavity communicating with an essentially cylindrical sprue opening at its upper end and (b) an elongated core pin within the cavity, cooperatively defining an essentially tubular, vertical mold cavity;   flowing heated thermoplastic material at relatively low pressure downwardly through the sprue opening and into the tubular mold cavity until both the cavity and sprue are essentially filled with thermoplastic material;   displacing the mold from the first station along a horizontal arcuate path about a vertical axis to a second station where the sprue opening is in vertical alignment with a vertically movable, essentially cylindrical pressurizing rod;   downwardly displacing the pressurizing rod into the sprue opening and applying pressure with the rod to the thermoplastic material while the material is cooled to a self-sustaining condition.   
     
     
       6. The method as defined in claim 5, including the further steps of: withdrawing the core pin from the elongated cavity with the thermoplastic material thereon;   stripping the thermoplastic material from the core pin;   inserting the core pin back into the elongated cavity; and   displacing the mold from the second station along an arcuate path back to the first station to receive additional heated, flowable thermoplastic material.   
     
     
       7. In a method of successively molding a plurality of articles of thermoplastic material, the steps of: (A) simultaneously, (1) positioning a first mold at a first station and (2) positioning a second mold at a second station, the first mold having an unfilled internal mold cavity and the second mold having an internal mold cavity essentially filled with heated thermoplastic material;   (B) simultaneously, (1) flowing heated thermoplastic material through a sprue in the first mold to fill the internal mold cavity thereof and (2) while the thermoplastic material in the second mold is cooled, applying pressure thereto by inserting a pressurizing rod into a sprue in the second mold and into contact with the thermoplastic material;   (C) removing the thermoplastic material from the mold cavity of the second mold in a self-sustaining condition; and   (D) simultaneously, (1) displacing the first mold away from the first station with heated flowable thermoplastic material therein and (2) displacing the second mold away from the second station for receiving additional heated flowable thermoplastic material.   
     
     
       8. The method as defined in claim 7, characterized in step (D) by displacing the first and second molds along respective arcuate paths, the first mold being displaced to the second station into alignment with the pressurizing rod and the second mold being displaced to the first station.